06PrecipitationEvaporation - Driver of precipitation...

10/16/2007ESM 203: Precipitation and evaporation1Driver of precipitationyPrecipitation mechanism is thus to cool air below its dew point, forming clouds in the presence of condensation nuclei: ◦water droplets or ice crystals ◦aerosols such as salt crystals and dust (subject to perturbation by humans)yDroplets coalesce and, when large enough, fall to ground.ySo ... how do we cool the air?◦Convection from underlying surface◦Mixing with colder air◦Both of these produce condensed water droplets but are not efficient enough to produce continuous heavy rain or snow◦Raising the air cools it rapidly enough to condense significant amounts of water vapor2Precipitation results from cooling of air to its Precipitation results from cooling of air to its dewpointdewpointtemperature in the presence of condensation nucleiyRising air encounters lower pressure so it expands5ddi b tipressure, so it expandsyExpansion requires that the air do work (expend energy) against the surrounding airyEnergy expenditure cools the airyIn Earth’s atmosphere, rising air cools by 1°C/100m, the dradiabatic lapse rate234ht, kmdry adiabaticambientdry adiabatic lapse rateyadiabaticmeans without the introduction of heat from external sources301-1001020T, °CVariation of atmospheric temperature with elevation reflects absorption of radiation emitted from surface and absorbed by atmospheric gasesyTemperature profile at any particular time and place may deviate dramatically from global averageyParticular rate of decrease is called the ambient atmospheric lapse rateyAverages– 0.65°C/100m, but varies tremendously, can even be 4positive (inversion).yVariations driven by recent history of mixing, conduction, and radiation Graedel, T.E. and Crutzen, P.J. (1995) Atmosphere, Climate and ChangeLapse rates in rising airyDry adiabatic lapse rate݀ܶ݀ݖൌ െ݃ܿ݌ൌgravityspecific heat of air at constant pressureൌ െ9.8msെ21005J kgെ1degെ1ൎ െ0.01degmെ1൫same as െ 10degkmെ1൯5yBut if water vapor condenses from the air during the cooling, latent heat is released, it warms the ascending airyWet (saturated) adiabatic lapse rate = Dry ALR + heat added by condensing water, –0.4 to –0.9 °C /100 m

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10/16/2007ESM 203: Precipitation and evaporation2Atmospheric stabilityyAir’s stability depends on the relationship between ambient atmospheric and 5ddi b tiadiabatic lapse rates◦If the ambient lapse rate is lower (more negative) than the dry ALR, the raised air is cooler (denser) than its surroundings◦Only way to rise is to be pushed up by some external agent (like a pressure-234ht, kmdry adiabaticambientagent (like a pressuregradient force pushing air over a mountain range) yMost stable is a temperature inversion601-1001020T, °CAtmospheric InstabilityyIf the ambient lapse rate is greater (less negative) than the 5dry ALR, the raised air is increasingly warmer (less dense) than its surroundings, and continues to risesy

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